This invention relates to direct current (DC) power sources and in particular to an apparatus and method for connecting a plurality of energy elements to achieve predetermined DC voltage steps and variations between those steps while ensuring uniform usage of energy elements.
The efficient operation of battery powered vehicles presents a number of unique problems. Firstly, it has been found that multiple discrete voltage steps are required by the DC motor in the vehicle during various phases of its operation. For instance, during the initial application of power to the DC motor, only a minimum voltage is required so as not to burn the motor. Thereafter, the voltage applied to the DC motor may be increased as the motor achieves its peak operating revolutions per minute (RPM).
The prior art has formulated different techniques to provide multiple discrete voltage steps to the DC motor. One such technique involves the use of a plurality of batteries connected in series in various numbers and configurations to achieve the desired voltage. For instance, assume that there are ten batteries in the power source, each having one volt in potential energy, and the DC motor requires two volts during the initial startup phase. By connecting two energy elements in series the desired voltage could readily be achieved. Additional energy elements are added to the two already connected as the DC motor proceeds to terminal RPM. However, this approach results in non-uniform discharge of the batteries since the batteries used in the startup phase are also used throughout the remaining phases of operation of the DC motor. This problem is compounded when the DC motor is subjected to numerous startup phases without attaining terminal RPM.
Non-uniform discharge of energy elements significantly diminishes the life of individual batteries or energy elements, particularly when dealing with batteries having a memory such as those manufactured based on nickel-cadmium technology. Non-uniform discharge also results in diminishing the duration of time that the vehicle can operate between recharges, since some of the energy elements (i.e., those not involved in the startup phase) will not be entirely discharged prior to the power source requiring a recharge.
Therefore, it would be advantageous if each of the energy elements were used in such a way that uniform discharge of each energy element was assured, thereby increasing the life of the individual energy elements and increasing the time between recharges.
Similarly, various voltage steps could be accomplished using a resistor ladder network. The network provides maximum resistance to one or more energy elements at startup resulting in minimum of voltage applied to the DC motor. Thereafter, the resistance is decreased yielding greater voltage to the DC motor. The disadvantage with this approach is the consequent loss of power in the form of heat dissipated by the resistors. Such a loss is particularly evident during startup, when the maximum number of resistors are used to reduce the voltage to its lowest value.
Therefore, it would be advantageous if energy were not lost during phases where the voltage applied to the DC motor must be reduced.
Another technique used to provide various voltage steps is achieved via modulation of the power to the DC motor. In this method the duration of the period in which voltage is applied to the DC motor is varied in order to achieve voltage steps. Several disadvantages result from this approach including heat loss in the DC motor and switching devices as well as degradation in the DC motor due to applying an essentially AC source of power to a motor designed for DC power.
Therefore, it would be advantageous if power was not significantly lost via heat in application to the DC motor while retaining the DC characteristics of the power regardless of the applied voltage.